1. Field of the Invention
The present invention relates generally to the assembly and testing of integrated circuit device components, such as multichip modules. Specifically, the present invention relates to a device and method for removably securing an integrated circuit device to a substrate and, in particular, to a spring contact for establishing a non-permanent electrical connection between a lead element of an integrated circuit device and a substrate.
2. State of the Art
Integrated circuit (IC) devices, such as Ball Grid Array (BGA) packages and Small Outline J-Lead (SOJ) packages, are commonly assembled into multichip modules for connection to higher-level packaging, such as a motherboard or a personal computer chassis. Generally, a multichip module (MCM) includes a carrier substrate, such as a printed circuit board, having a plurality of IC devices mounted thereto. Other electrical components, such as resistors, capacitors, inductors, or other suitable devices, may also be mounted on the carrier substrate of the MCM, or even on the IC devices. Electrical communication among the IC devices, between the IC devices and other electrical components on the multichip module, and between the devices and external components is established by conductors on the MCM carrier substrate. The conductors may be conductive traces fabricated on the surface of, or internal to, a printed circuit board. Methods for fabricating printed circuit boards having conductive traces, as well as other types of substrates having conductors thereon, are well known in the art.
Conventional IC devices, such as BGA, TSOP, SOP, SOJ, etc. packages, generally comprise a semiconductor die electrically connected to a plurality of electrical leads that is encased within an encapsulant material, a portion of each of the electrical leads extending from the encapsulant material and configured for establishing electrical connections between the semiconductor die and external components or higher-level packaging. An exemplary embodiment of a conventional BGA package is shown in FIGS. 1 and 2. The conventional BGA package 10 includes a semiconductor die 20 secured to a die-attach pad 35 formed on an upper surface 31 of a substrate 30, which may also be termed an “interposer.” The BGA package 10 also includes a plurality of electrical leads 40 adapted to provide electrical communication between the semiconductor die 20 and one or more external components (not shown). The semiconductor die 20 and at least a portion of each electrical lead 40 may be encased by an encapsulant material 50. The conventional BGA package 10 may be a memory device, such as a DRAM chip, a processor, or any other integrated circuit device known in the art.
Each of the electrical leads 40 includes an external conductive ball (or bump, pillar, or other lead element) 41 configured for electrical connection to an external component. The conductive ball 41 may be secured to a conductive pad 42 formed on a lower surface 32 of the substrate 30. Each electrical lead 40 further comprises a conductive via 43 extending from the conductive pad 42 and through the substrate 30 to a conductive trace 44. The conductive trace 44 (only a few of which are shown in FIG. 1 for clarity) is formed on the upper surface 31 of the substrate 30 and provides an electrical path from the conductive via 43 to a bond end 45 located proximate the semiconductor die 20. A bond wire 46 attached to the bond end 45 of the conductive trace 44 and extending to the semiconductor die 20, where the bond wire 46 is attached to a bond pad thereon, electrically connects the electrical lead 40 to the semiconductor die 20. At least the bond wire 46 and conductive trace 44 of each electrical lead 40 may be encased by the encapsulant material 50.
The conventional BGA package 10 may include a plurality of the conductive balls 41 arranged, for example, in an array or arrays of mutually adjacent rows and columns. Referring to FIG. 1, the conductive balls 41 may be arranged in two arrays 60, 70, each array 60, 70 disposed between an edge of the semiconductor die 20 and a peripheral edge of the substrate 30. Each array 60, 70 comprises three columns 61, 62, 63, 71, 72, 73, respectively, of conductive balls 41. The arrangement of conductive balls 41 is typically referred to as the “pin-out” or the “footprint” of the BGA package 10. Those of ordinary skill in the art will understand that the particular pin-out of the BGA package 10 may vary depending upon the application and that the pin-out may be of any suitable configuration.
To attach and electrically connect the conductive balls 41 of the BGA package 10 to a substrate, such as, for example, an MCM carrier substrate or a burn-in board, the substrate is configured with a plurality of contact pads arranged in a number of contact pad arrays. Each contact pad array includes a number of contact pads arranged in a pattern corresponding to the pinout of the BGA package 10. The conductive balls 41 of the BGA package 10 may be formed of solder or a conductive or conductor-filled epoxy. If solder, the conductive balls 41 are reflowed to connect to the contact pads of the contact pad array on the substrate. If epoxy, the conductive balls 41 may be first heated to a tacky “B” stage to adhere to the contact pads, and then further heated to completely cure the epoxy to a “C” stage. A substrate may include a plurality of IC devices mounted thereto, wherein each of the IC devices is permanently attached to a corresponding contact pad array on a surface of the substrate. By way of example, an MCM may be a memory module comprised of a one-piece carrier substrate having opposing surfaces, with one or both of the opposing surfaces of the carrier substrates including multiple contact pad arrays and a plurality of IC devices, such as BGA and SOJ packages, mounted thereto.
During the fabrication of an IC device, the IC device may be subjected to individual component-level testing, such as burn-in and electrical testing. An IC device that exhibits a desired level of performance during component-level testing is generally referred to as a “known good device” or “known good die,” while an IC device failing to meet minimum performance characteristics may be referred to as a “known bad device.” After component-level testing, the IC device may be assembled into higher-level packaging, such as an MCM, and again subjected to testing. Testing of higher-level packaging such as an MCM, referred to herein as module level testing, may include burn-in, electrical characterization and performance evaluation, as well as other desired electrical testing.
If an MCM fails to exhibit minimum operating characteristics during module level testing, an IC device causing the failure, which may have previously been identified as a “known good device” during component-level testing, must be removed from the MCM and replaced. Also, it may be desirable to introduce a “known bad” IC device into an MCM for module level testing in order to observe the electrical characteristics of the MCM with the “known bad” IC device, or to observe the electrical characteristics of the “known bad” IC device at the module level. After module level testing is complete, the “known bad” IC device must be removed from the MCM and replaced. Thus, although individual IC devices are typically tested at the component level, it is desirable to subject IC devices to further testing at the module level, as a “known good device” may fail at the module level and, further, because incorporation of a “known bad device” into an MCM may be useful in module level testing.
To test IC devices in a higher-level environment, module level testing is generally performed after the IC devices are assembled into and permanently attached to, for example, an MCM carrier substrate. Thus, if an IC device must be removed from an MCM after module level testing, the permanent electrical bonds between the electrical leads of the IC device, for example, the conductive balls 41 of the conventional BGA package 10, and the contact pads on the MCM carrier substrate must be severed. Severing these permanent electrical bonds, which, as noted previously, typically comprise solder or conductive epoxy, may cause both heat-induced and mechanical damage to the MCM carrier substrate and conductors, to the electrical leads and electrical bonds of the IC devices remaining on the MCM, and to other electrical components mounted on the MCM.
Also, it may be necessary to remove an IC device from a substrate to achieve an upgrade. For example, as technological advances are made by IC device manufacturers, it is often desirable to replace an IC device mounted to a substrate with a next-generation IC device exhibiting improved performance characteristics. To replace an obsolete IC device mounted to a substrate, such as the carrier substrate of an MCM comprising part of, for example, a personal computer, the permanent electrical bonds between the electrical leads of the obsolete IC device and a plurality of contact pads on the substrate must be severed, which may cause both heat-induced and mechanical damage to the substrate and to other IC devices remaining on the substrate.
To prevent heat-induced and mechanical damage resulting from severing of the permanent electrical bonds between the electrical leads of an IC device and a plurality of contact pads on a substrate, the IC device may be non-permanently attached to the substrate for module level testing, as well as for final assembly. Use of non-permanent connections between the electrical leads of an IC device and a contact pad array of a substrate allows for easy removal of the IC device after module level testing or after final assembly without any resulting damage from the severing of permanent electrical bonds. Sockets and fixtures for non-permanently attaching an IC device to a substrate are well known in the art; however, such sockets can be relatively expensive and their cost often does not justify their use. Although the cost of conventional sockets and fixtures may, in some instances, be acceptable for limited use applications, such as testing and small production runs, their cost is generally not acceptable for full-scale production.
Use of non-permanent electrical connections between the electrical leads of an IC device and a contact pad array of a substrate can, however, itself cause problems during module level testing and/or at final assembly. Non-planarities in the substrate, in the conductors forming a contact pad array, or in the IC device itself, may, in the absence of a permanent bonding agent, result in poor electrical contact between an electrical lead of the IC device and a corresponding contact pad on the substrate. For example, non-planarities in the substrate 30 of the BGA package 10, as well as inconsistency in size of the conductive balls 41, may result in unreliable electrical contact between the conductive balls 41 and the contact pads of a substrate in the absence of a permanent bonding agent. Similarly, for other types of IC devices, such as the SOJ package, deflection of their electrical leads as they come into contact with the contact pads on the substrate may, again, in the absence of a permanent bonding agent such as solder or conductive epoxy, result in poor electrical contact. Poor electrical contact resulting from non-planarities and/or lead deflections may produce unreliable test data during module level testing or prohibit the acquisition of any meaningful test data, and such poor electrical contact may result in non-functional assembled IC device components or assembled IC device components which do not comply with customer or industry specifications. In addition, the use of non-permanent electrical connections between the electrical leads of an IC device and a contact pad array may result in an overly long electrical path with increased inductance, a characteristic which degrades signal integrity at high device operational speeds.
Therefore, a need exists in the art for a low-cost device and method of forming non-permanent electrical connections between the electrical leads of an IC device and a contact pad array of a substrate. Such an apparatus and method must also provide for robust and reliable electrical connections between the electrical leads of an IC device and the contact pads on a substrate.